Development of Paste-to-Paste-type Phosphate-bonded Investments

Objective: It is well-known that dental investments are composed of fine powders. However, they may cause a respiratory disease if laboratory workers absorb them by mistake when opening the package, weighing and initially mixing at work. The objective of this study is to produce paste-to-paste type of phosphate-bonded investments without dust. Methods: Two pastes (Code: paste1 & paste2) were prepared prior to test. Paste1 was a mixture of cristobalite (average size; 9.2µm, Tatsumori Cor.) and the phosphate (Magnesium dihydrogenphosphate; Yoneyama chem.) liquefied with distilled water of 20cc. Paste2 was MgO (Magnesium oxide #220, Tateho chem.) coagulated by oily compounds. Four (A-D) experimental phosphate-bonded investments (paste1(g)/paste2(g) = A: 79.6/25.5, B: 82/22.5, C: 84.4/19.5, D: 86.8/16.5 ) were designed and their fundamental properties were evaluated (n=4-5). Results: Paste2 was effective to harden the mold and shorten setting time. Compressive strength of A showed 8.65±1.55MPa as green strength and 7.60±0.62MPa as fired strength. A was obviously strongest in the experimental molds. These strength values became smaller alphabetically, and there were significantly differences among A-D by One-way ANOVA (p<0.05). On the other hand, setting time of A was about 8 minutes and shorter than those of B-D. D was found to set after about 13 minutes from mixing. Thermal expansion is depended on the amount of cristobalite including in the mold. In this study, D containing more paste1 was advantageous on thermal expansion. The expansion rate was about 1.8% at 900°C. Conclusions: These results suggest that experimental paste-to-paste type phosphate-bonded investments are useful for operators because of complete dust-free and usable properties as dental casting molds. Hence, we need to explore more suitable paste/paste ratio for dental casting molds or evaluate castings obtained from them.